The present invention relates generally to a silent chain, and more particularly to a so-called xe2x80x9crocker jointxe2x80x9d type silent chain having a large number of link plates interleaved and articulately connected by rocker joint pins.
Rocker joint type silent chains are known in which the pitch of guide plates and the pitch of link plates are made equal to each other for the purpose of distributing the loads evenly over the link plates arranged in the widthwise direction of the chain to thereby lengthen the service life of the chain.
According to one conventionally proposed improvement, the pitch of the guide plates is initially made smaller than the pitch of the link plates, and by appropriately changing the shape, thickness, material or hardness of the guide plates, the guide plates are made susceptible to plastic deformation when subjected to a proof load. Thus, after the proof load is applied to the guide plates, the pitch of the guide plates becomes substantially equal to the pitch of the link plates.
In another conventionally proposed improvement, the pitch of the guide plates is initially made greater than the pitch of the link plates, and the pitch of the link plates is enlarged when a proof load is applied to the link plates to such an extent that the pitch of the link plates and the pitch of the guide plates are substantially equal to each other.
According to still another conventionally proposed improvement, a longer pin and a shorter pin initially formed in a bent or curved configuration are used in combination so that when the silent chain is subjected to a tensile force, the bent pins perform a spring action effective to reduce the noise and wear of the silent chain.
When a silent chain is actually used in an automobile engine or an automobile transmission, a tensile force acting on the silent chain also acts on the guide plates. Accordingly, the conventional silent chains according to the first and second improvements described above are likely to deteriorate in durability and have a shortened service life because the guide plates are reduced in strength and hence likely to be broken due to plastic deformation caused when the proof load is applied to equalize the pitches of the guide plates and link plates before the actual use of the silent chain.
The silent chain according to the third conventional improvement described above also encounters a difficulty that, since the bent longer pin used in combination with the shorter pin forms a space at each end of the rocker joint pin, the rocker joint pin cannot be easily inserted through the inner link plates and thus lowers the assembling efficiency of the silent chain. In order to achieve the desired assembling efficiency, an attempt may be made to enlarge pin holes formed in the link plates. However, the attempted enlargement of the pin holes will result in a reduction of the strength of the link plates.
It is accordingly an object of the present invention to provide a rocker joint type silent chain including structural features which are capable of increasing the strength of guide plates and link plates and also reducing the operation noise of the silent chain.
According to the present invention, there is provided a silent chain comprising: guide link rows each having a pair of guide plates and at least one link plate disposed between the guide plates; joint link rows each having link plates numbering one more than the number of the link plates in each of the guide link rows; and rocker joint pins connecting the guide link rows and the joint link rows alternately in an endless fashion. Each of the rocker joint pins is composed of a longer pin and a shorter pin having a length smaller than the length of the longer pin. The longer pin is fitted in each of a pair of pin holes formed in each of the guide plates and also is inserted through each of a pair of pin holes formed in each of the link plates, and the shorter pin is inserted through each of the pair of pin holes formed in each link plate. The longer pin and the shorter pin are initially formed in a straight configuration and have respective rocker surfaces in contact with each other. When the silent chain is stretched substantially straight after having been subjected to a proof load, the pitch of the guide plates is 0.2% to 0.7% greater than the pitch of adjacent contact points between the longer pins and the shorter pins of the rocker joint pins in the guide link rows so that the longer pins, which were initially formed in the straight configuration, are elastically deformed into a bow-like configuration bent toward the mating shorter pins.
The term xe2x80x9cproof loadxe2x80x9d is used herein to refer to a predetermined load applied to a silent chain after assembly so as to remove an initial elongation caused due to plastic deformation such as burrs and assembling distortion that are unavoidable for the guide plates and link plates.
After application of the proof load, the silent chain is stretched substantially straight. In this condition, the pitch of the guide plates is made slightly greater than the pitch of the link plates so that the longer pins initially formed in a straight configuration are elastically deformed into a bow-like configuration bent toward the mating short pins. With the longer pins thus deformed elastically, the rocker joint pins, and hence the silent chain, as a whole, acquire spring properties.
When the silent chain is used in an automobile engine or an automobile transmission, the guide plates are pre-loaded by a compressive force exerted from the elastically deformed longer pins. Accordingly, when the chain tension is small, the chain tension is exclusively borne by the link plates. As the chain tension increases, the link plates undergo elastic deformation in the longitudinal direction of the chain, which gradually lessens the compressive force of the longer pins. Subsequently, the pitch of the guide plates becomes substantially equal to the pitch of the link plates whereupon the guide plates start to bear the chain tension.
By thus applying a compressive pre-load to the guide plates, and by setting the difference in pitch between the guide plates and the link plates to be not greater than the amount of elastic deformation of the link plates in the longitudinal direction of the silent chain, it is possible to change the magnitude and direction of loads on the guide plates according to the chain tension. When the chain tension is small, the link plates bear the chain tension. Conversely when the chain tension is large, the guide plates bear the chain tension in cooperation with the link plates.
The pitch difference is preferably in the range of 0.2 to 0.7% of the pitch of the link plates. If the pitch difference is less than 0.2%, it is difficult to bend the longer pin arcuately toward the mating shorter pin due to cumulative tolerances of the parts (i.e., the guide plates, link plates and rocker joint pins). Thus, the rocker joint pins do not acquire spring properties and, hence, they are unable to exert a compressive pre-load onto the guide plates. If the pitch difference exceeds 0.7%, the longer pins are bent into an excessively large curvature, tending to cause a breakage of the longer pins as well as excessively large compressive pre-loading of the guide plates.